Evolution Process and Multi-scale Structure of a Polar Low Developed over the Japan Sea on 11∼12 December 1985

Part 1: Evolution Process and Meso-α-scale Structure

Abstract

The evolution process and structure of a polar low of the meso-α-scale, which formed over the Japan Sea on 11 December 1985 are studied. In Part I, the evolution process and the meso-α-scale structure of the polar low are studied. In Part II, the meso-β-scale fine structure of the polar low is studied.
An important synoptic-scale setting for the polar low formation is the low-level cyclonic polar air streams over the Japan Sea, which are induced by the development of a synoptic-scale low over the northwestern Pacific near Japan.
In association with the passage of a meso-α-scale cold vortex aloft over the Japan Sea a "moist dome" (dome-shaped polar air of moist neutral lapse rate) forms over the Japan Sea. At the same time, a meso-α-scale low (polar low) develops and moves south-eastward. This polar low has a warm core structure in the 900∼700 mb layer and well-organized spiral cloud bands. The cyclonic circulation of the polar low is limited in the lower troposphere.
The 24-hour numerical prediction of the present polar low by a meso-scale primitive equation model is examined by comparing with observations. The formation process and the meso-α-scale structure of the polar low are well predicted by the model from the initial state 12 hours before the polar low formation.